It is because there is not a very good carry over.
About a 2 years ago there was this guy who would come in the gym and make a huge deal out of his leg press.
He would wrap his knees, get all worked up, pile a bazillion plates on the machine, and then move the weight about 4 inches a bunch of times with his hands on his knees, and make a big show of slamming it on the catch when finished. He had a little butt buddy who followed him around like Boo-Boo Bear.
If we were lucky enough to be squatting on the same day, he would cast a lot of tough guy glances our way, and seemingly make it a point to walk past us as part of his big psyche up routine.
Anyway, one day I was completely sick of him and as he was walking past I said, "You are the first person I have ever seen make such a big deal out of leg presses. Are you training for the leg press Olympics?"
He got kind of flustered and I then followed up with , "If you have to get that worked up to leg press, you must be absolutely scared to death of a barbell on your back."
He mumbled something about a bad back, of course, to which I told him I would be absolutely shocked if he could perform one full range of motion rep with 225.
He chose not to take me up on it. I haven't seen him since, don't know if that is a coincidence or not.
Moral of the story, as Captain Kirk once said, and I am paraphrasing, the only way to build a big squat is to squat.